This invention relates to a sewing machine which sews upper and lower cloth pieces having the same pattern together while maintaining correct alignment of the pattern.
Generally, in a conventional sewing machine, a skilled worker must put many hours and take great care in sewing together two cloth pieces having the same pattern to form the back body of a blazer for either a gentleman or a lady or to fasten the sleeves to the shoulders of the blazer body while maintaining correct alignment of the pattern.
U.S. Pat. No. 3,954,071 to Mall et al. discloses a sewing machine for sewing two material layers to each other in aligned positions. According to this sewing machine, although it is possible to counteract the misalignment of the material layers, it is impossible to perform the pattern-correct sewing together of two cloth pieces having the same pattern.
U.S. Pat. No. 4,612,867 to Rosch et al. discloses a sewing machine for sewing together two pieces of cloth while maintaining correct alignment of a pattern on the two pieces of cloth. In this sewing machine, a light source and a light sensor are provided for each of the upper and lower cloth pieces, and the mutual difference of the pattern of the two cloth pieces is determined by comparing sensing signals from each light sensor. However, this sewing machine is disadvantageous in that sensors must be provided for each of the upper and lower cloth pieces and a long time is required from the occurrence of the difference of the pattern to the disappearance of the difference.
The reason a long time is required is as follows: In this sewing machine, each sensor detects each pattern structure of upper and lower cloth pieces and the detected signals are converted to digital signals and are stored. A signal processing unit determines a momentary offset between the two cloth pieces and the direction of the offset based on the stored digital signals for each of the upper and lower cloth pieces by using cross-correlation function. In general, this operation of the signal processing unit requires a long time. Also, in this sewing machine, there is another problem, that is, the signal processing unit can not detect the offset until the pattern structures are fully detected.